This invention relates to multi-wall paper sacks, and to powder products packaged in such sacks.
It is common practice for finely divided powder products to be packaged in multi-wall paper sacks provided with a filling opening--a so-called valve--at one end. To fill the sack, a filling nozzle is inserted into this opening and the powder is delivered from this nozzle into the sack. If the filling machine is operated at high speed it can be difficult for air to vent from the sack while it is being filled.
Some powder products, notably plaster and cement, react with water when used, and require protection from atmospheric moisture during storage. For this purpose it is conventional for a multi-wall sack to incorporate a vapour barrier between the walls of the sack, for instance a coating of polyethylene on the outer surface of an inner wall or on the inner surface of an outer wall.
It is possible to provide some perforations through sack walls to allow air to vent while the sack is being filled, even though these perforations penetrate a vapour barrier. This has been done in Europe and a form of sack which has been used extensively on high speed filling machines in United Kingdom has two walls with a vapour barrier on the outside of the inner wall; there are perforations through the wall and this vapour barrier over the whole of its area and a small number of perforations through both walls near the top of the sack.